Vinagre bookmarks are stored in the file vinagre-bookmarks.xml file under /home/
/.local/share/vinagre directory.
As all the bookmarks are stored in single line be bit more cautious before editing, otherwise you will endup mess.
(I Recommend to keep a backup of the file before editing)
All the bookmarks are starts and ends with - &
xml node. So as per your wish do edit/modify the Bookmarks.
Example entry in vinagre-bookmarks.xml file:
<![CDATA[
<item><item><protocol>vnc</protocol><name>Asset-Mgmt-Client-1</name>
<host>10.184.48.82</host><username></username><port>5915</port>
<fullscreen>0</fullscreen><width>800</width><height>600</height>
<view_only>0</view_only><scaling>0</scaling><keep_ratio>1</keep_ratio>
<depth_profile>0</depth_profile><lossy_encoding>0</lossy_encoding></item>
]]>
